The measurement of systolic, mean, and diastolic blood pressures, referred to herein collectively as “blood pressure,” is an important and well-known diagnostic procedure in animal medicine, and especially in human medicine. Blood pressure may be measured either directly, by insertion of a pressure-measuring catheter into an artery, or indirectly (inferentially), usually by means of an inflatable cuff. In using a cuff, the operative principle is that pneumatically inflating the cuff compresses an underlying artery in the subject, and the respective pressures are determined as air pressure is gradually released from the cuff.
In design, known cuffs are either open or closed. An open cuff is a flat device which is wrapped around the subject's extremity. A closed cuff is a generally cylindrical device through which a subject's extremity is inserted.
In operation, a typical cuff is applied to encircle an upper arm, thigh, or in some cases, a wrist or forearm. The cuff is then inflated to an indicated cuff air pressure at which the flow of blood through the underlying artery is stopped, such air pressure being greater than the subject's systolic pressure. Pressure within the cuff is then gradually reduced. When blood flow through the artery resumes, the cuff pressure corresponds to the subject's systolic pressure. When pressure in the cuff drops below the subject's diastolic pressure, blood flows in the artery through the complete cardiac cycle. Optionally, mean blood pressure may also be determined.
These flow phenomena can be determined in any of several ways: for example, a) by listening via a stethoscope to the sounds emanating from the artery downstream of the cuff; b) by observing the relatively small changes in pressure in the cuff caused by the encircled artery's opening and closing during the cardiac cycle; or, c) by using a Doppler flowmeter to measure flow rates. Each of these methods is well known in the art.
There are several potential sources of error in determining blood pressure using a prior art inflatable open cuff. An important consideration is that the dimensions of the cuff be selected to fit the subject to be tested. The contact dimension of a cuff in a direction along a subject's arm is referred to herein as the cuff contact width, and the contact dimension circumferential of the relaxed arm is the circumference, or contact length. Preferably, the contact width of the cuff is about 40% of the arm circumference, and the contact length of the inflatable part of the cuff is at least 70% of the arm circumference. If the total contact area of the cuff is too narrow, blood pressure is over-estimated; if the area is too wide, pressure is under-estimated.
In common medical practice, either a wrap-around cuff of proper dimensions or a closed (cylindrical) cuff of fixed dimensions is used. In stand-alone uses today (stand-alone being defined as not being part of an automated blood pressure machine), a “wrap-around” cuff is typically employed. Such a cuff comprises an elongate flat fabric pocket formed of non-elastic material into which a distensible bladder is inserted for pneumatic inflation. The cuff is wrapped snugly around the subject's extremity, such as an arm or leg, and is then secured in place, for example, by Velcro attachments. (As used hereinbelow, the term “arm” should be taken generally to mean any of the various anatomical extremities or appendages in which blood pressure may be measured with a cuff.) Typically, the bladder is not attached to the fabric of the cuff and thus is free to conform to each subject's arm in both contact width and contact length.
Because proper contact length is important for accurate blood pressure measurement, the circumference of a subject's arm should be determined before attempting blood pressure measurement with an open cuff. In good medical practice, the circumference of the subject's arm determines the size cuff to be used. For adult humans, a choice of open cuffs may include three sizes standard in the art: Small Adult, Normal Adult, and Large Adult. Each cuff is sized for an arm circumference in the middle of a range which overlaps the range of an adjacent cuff. For example, a Normal Adult cuff may be specified for use on arm circumferences of 25–35 cm, and a Large Adult cuff may be specified for circumferences of 33–47 cm. Either cuff may be used on a 34 cm arm, but neither cuff is optimal because the contact width of the cuff is optimized for the middle of each range. Blood pressure measurements of a given subject using the two different cuffs may differ by several mmHg. Of course, a greater number of different cuffs, each having a narrower range of arm sizes, would lessen this error but would require a large and unwieldy inventory of open cuffs at each measurement site.
Unfortunately, it is common in stand-alone use to ignore the benefits of sizing the cuff to the subject and to simply use one cuff for all subjects, with concomitant sacrifices in accuracy. What is needed in the stand-alone art is a single cuff that can provide accurate blood pressure measurement over a very wide range of arm sizes.
Closed cuffs also present a serious problem in use. A principal prior art use for closed cuffs is in automated measurement machines. Such machines are intended to make correct measurements on the entire population of human users using a single cuff. Because the cuff is closed, the nominal outer and deflated inner diameters are fixed at manufacture to accommodate the largest arm intended for measurement, and the inflation means is attached to the fabric of the pocket. Therefore, the cuff must expand radially inwards first to occupy the slack space necessary to permit insertion of an arm into the cuff, and then further to properly compress the inserted arm.
The main source of error in using an oversize closed cuff arises from severe puckering of the cuff material. In the prior art, an inflation chamber is formed by heat sealing and then stitching or gluing together two air-impermeable layers of non-distensible, non-elastic material along all four edges thereof. An outer material shell and seam tape are also stitched into the cuff, making the cuff relatively stiff and non-compliant. As the chamber is inflated, the inner layer is forced into relatively large radial puckers or wrinkles, and especially so if the subject's arm is relatively small in circumference. These puckers constitute breaks in the encircling pressure and can lead to outright failure of the measurement when a pucker is formed over the brachial artery being tested. Further, chamber pressure significantly higher than systolic may be required to close the puckers and the underlying artery, thus giving a falsely high measurement of systolic pressure. Further, because the chamber layers are constrained along all four edges, the inflation chamber cannot readily collapse transversely to form a desirable oval cross-sectional shape.
What is needed is an improved blood pressure closed cuff that can provide accurate measurement in an automated blood pressure machine over the full range of arm sizes of potential human users.
It is a principal object of the present invention to make accurate measurements of blood pressure over a wide range of human arm sizes using a single cuff.
It is a further object of the invention to make such measurements via an automated blood pressure machine.
It is a still further object of the invention to make such measurements in stand-alone use.